This invention pertains to a process for producing a reaction product via equilibrium-limited reactions, such as esterification and alcoholysis (or transesterification) reactions, wherein the process is conducted using two reactor stages and the desired product or products of the equilibrium-limited reactions are removed in the vapor phase, preferably from the second stage reactor.
Equilibrium-limited reactions generally involve the reaction of two or more reactants to produce at least one product and, typically, a coproduct. In order to achieve a greater conversion to the desired product(s), various techniques have been suggested such as removing the coproduct and/or product from the reaction menstruum to maintain a driving force toward the product.
Equilibrium-limited reactions are often conducted in a single reactor with product being selectively removed from the reaction menstruum or in a plurality of reactors in which product is separated from the reaction menstruum in each of the reactor stages. One type of multistage reactor process is disclosed in U.S. Pat. Nos. 5,811,574 and 5,900,125, which disclose a process and an apparatus for the continuous preparation of alkyl esters of (meth)acrylic acid by reacting (meth)acrylic acid and monohydric alkanols of 1 to 8 carbon atoms in the homogeneous, liquid, solvent-free phase at elevated temperatures and in the presence of an acidic esterification catalyst. In the disclosed process, the (meth)acrylic acid, the alkanol and acid catalyst are continuously fed to a reaction zone which consists of a cascade of at least two reaction regions connected in series, and the discharge stream of one reaction region forms a feed stream of a downstream reaction region. The cascade may have from two to four reaction regions spatially separated from one another. These patents disclose an aqueous azeotropic distillation process in which the target alkyl acrylate formed in the reaction zone is separated from the catalyst and starting acid via the top of a rectification zone mounted on the reaction zone as a component of at least one azeotropic mixture consisting of water or water and starting alkanol as further components in addition to the alkyl acrylate. The resulting distillate is separated into at least one organic phase containing the alkyl acrylate and into at least one water-containing aqueous phase. A part of the organic phase containing alkyl acrylate is recycled via the top of the rectification zone. The remaining organic phase, which contains no starting acid, is fed into downstream equipment to isolate the target alkyl acrylate from starting alkanol and other impurities having lower and higher boiling points than the target ester. It is also disclosed that this process is preferably to be used for the preparation of n-butyl acrylate because the boiling points of n-butyl acrylate and acrylic acid are comparatively close together. It is further disclosed that the water content in the water/alkyl acrylate or water/alkanol/alkyl acrylate azeotropic mixtures increases along with the increase of the molecular weight of the alkyl acrylate. Higher energy usage and larger equipment are needed if this azeotropic distillation process is used for the preparation of higher molecular weight acrylates, such as 2-ethylhexyl acrylate.
U.S. Pat. No. 5,883,288 discloses a process for the continuous preparation of alkyl esters of (meth)acrylic acid by reacting (meth)acrylic acid and monohydric alkanols of 1 to 8 carbon atoms in the presence of an acidic esterification catalyst in a reaction zone. The reaction zone can consist of a cascade of at least two reaction regions connected in series, and the discharge stream of one reaction region forms a feed stream of a downstream reaction region. The cascade may have from two to four reaction regions spatially separated from one another. A product mixture is discharged from the reaction zone and fed to a rectification unit (I) and separated into at least one product comprising the alkyl ester of (meth)acrylic acid and one product comprising the catalyst. The alkyl ester of (meth)acrylic acid product is then fed to a further rectification unit (II) and separated off by rectification.
The disadvantage of such a process is that the acid catalyst is sent with the reaction mixture from the reaction zone to a rectification unit (I), rather than being allowed to remain in the reaction zone. The combination of high acid catalyst concentration and high temperatures in the lower section and the bottom of the rectification unit (I) lead to high rates of corrosion, equipment fouling, and undesired side reactions, such as decomposition of the acid catalyst.
Accordingly, an improved process for conducting equilibrium-limited reactions is sought that would minimize and/or eliminate the problems associated with conducting the reactions and with recovering unreacted reactants and product.